Radiotelephone system with the character of a local or auxiliary communications apparatus

ABSTRACT

A transmission process with specific adaptive properties in an analog or mixed analog-digital transmission system, having a fixed base station and a number of subscriber station independent of one another. Information from the base station is transmitted to the subscriber stations wherein each information channel is converted by suppressed carrier, single-side band modulation to a narrow-band signal. The narrow-band signals are arranged spectrally adjacent one another in a frequency multiplex allocation scheme and are modulated on a carrier set at a higher frequency range using phase or frequency modulation. Each subscriber accesses no more than one of the information channels. Information transmissions from the subscriber stations to the base station is performed using narrow-band radio channels.

BACKGROUND OF THE INVENTION

I Field of the Invention

The present invention relates to a radio transmission system forcommunications between a fixed station and a number of fixed subscriberstations that operate independently of one another.

II Description of the Known Art

For information transmission in systems with a radial operatingstructure which are characterized by a local base station and aplurality of noninterdependent subscriber stations, wherein thetransmission of information is performed each time from the base stationto the individual subscriber stations and from the subscriber stationsto the base station, different transmission processes are used.

It is characteristic of such systems that, as a rule, access by theindividual subscriber stations is limited to no more than one of a greatnumber of information channels at a time. These information channels arecombined by multiplex processes. Code-multiplex, time-multiplex andfrequency-multiplex processes are known.

In the code-multiplex process the information is modulated by basemodulation on a carrier, and the resultant signal, which is anarrow-band signal in comparison to the channel bandwidth, is spread outspectrally to the channel bandwidth by multiplex modulation by means ofa code word identifying the receiver.

The recognition of the signal is performed with the aid of the spectralcoding. The message is selected in the receiver from among the pluralityof spectrally coded messages superimposed in the code multiplex channel,with the aid of the code word associated with it.

In the time-multiplex process the entire bandwidth of a radio channel isavailable to each subscriber, which the latter may use only at definite,short time intervals. The characters or groups of characters identifyingthe different subscribers are contained one inside the other and aretransmitted at a correspondingly higher bit rate in the single radiochannel, while the time channel associated with each subscriber isrepeated periodically with the length of the frame period.

Code-multiplex and time-multiplex processes are used preferentially fordigital information transmission systems.

By their use virtually any signal-to-noise ratio can be achieved inexchange for a great transmission bandwidth.

The code- and time-multiplex processes require very efficient, and atpresent still expensive digital signal processing, especially in thesubscriber stations, but also in the base station. In the time-multiplexprocess the necessary line synchronization is complicated, and, in thecode-multiplex process especially, so is the code synchronization in thesubscriber stations.

In the frequency-multiplex process the total bandwidth available for thetransmission of information is divided up into narrow frequency bandswhich correspond each to one information transmission channel. Such anarrow frequency band is available to the subscriber throughout theradio transmission. Frequency-multiplex processes can be used toadvantage in analog or combination digital-analog systems requiring onlymedium quality because an established cost-effective technology isavailable and only a relatively small transmission bandwidth is needed.

For the use of the above-described multiplex transmission processes inmost of the systems of this kind known heretofore, especially radiotransmission systems, the same processes have been used in bothdirections of information transmission, i.e., both from the base stationto the subscriber stations and also in the opposite direction.

Such a system is described, for example, in the German patentspecification DE-OS 2537683. The individual radio channels are operatedas duplex channels in the lower and upper sideband with a given duplexseparation, all channels being disposed in the frequency multiplex andare all at the same channel separation from one another.

Likewise, transmission processes of this kind are known from carrierfrequency technology and from radio beam technology. In contrast to thesystems herein considered, however, a point-to-point informationtransfer is performed between two fixed base stations, a relativelygreat number of information channels being always beamed intransmission.

German patent specification DE-OS 3447107 discloses a digitalinformation transmission process for cellular networks, in which adifferent modulation process is used in each direction in theinformation transmission channel. For the transmission of informationthe mobile subscribers access one of a number of information channels.

In the direction from the fixed base station to the subscriber stations,each information channel is spread by spread spectrum modulation. Thespread information channels are superimposed on one another and thebroad-band sum signal thus obtained is transmitted in a common frequencyband.

The transmission of information from the subscriber stations to the basestation is performed in narrow-band frequency channels separate from oneanother.

Code multiplex is favored as a multiplex process and is combined with abinary spread modulation. This process has advantages especially inregard to security against interference in mobile radio networks. Thespread modulation that is concretely to be used can be varied beforetransmission starts.

Various measures, such as the use of different kinds of spreadmodulation, different frequency bands, different time channels, limitedchannel numbers and channel frequencies serve for interferencesuppression between adjacent radio cells.

German patent specification DE-OS 3527331 discloses a transmissionprocess which uses different combinations of multiplex processes in bothdirections of transmission of the digital radio system for the beamedtransmission of information channels.

In the base station to subscriber station transmission the informationto be sent is inserted into the channel using the combination ofcode-multiplex, time-multiplex and frequency-multiplex processes whichis demultiplexed by the same process in the subscriber station.

Transmission from the subscriber stations to the base station isperformed in narrow-band frequency channels separate from one another,or in time channels, or in code planes separate from one another, orcombinations thereof.

When traffic is limited, beaming methods are proposed, with theexclusive use of time-multiplexing or a combination of time-multiplexingand code-multiplexing.

Conversion to carrier frequencies is also being discussed.

SUMMARY OF THE INVENTION

The present invention aims to use a transmission process with specificadaptive properties in an analog or mixed analog-digtal radiotransmission system, having a fixed base station and a number ofsubscriber stations independent of one another.

According to the invention, information from the base station istransmitted to the subscriber stations wherein each information channelis converted by suppressed carrier, single-side band modulation to anarrow-band signal. The narrow-band signals are arranged spectrallyadjacent one another in a frequency multiplex allocation scheme and aremodulated on a carrier set at a higher frequency range using phase orfrequency modulation. Each subscriber station accesses no more than oneof the information channels. Information transmission from thesubscriber stations to the base station is performed using narrow-bandradio channels.

For a better understanding of the invention, reference is made to thefollowing description taken in conjunction with the accompanyingdrawing, and the scope of the invention will be pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing:

FIG. 1 is a frequency spectrum of a telephone signal being transmitted,

FIG. 2 a frequency spectrum of the an organization or management signal,

FIG. 3 a spectrum of the modulated signal for transmission from the basestation to a subscriber station,

FIG. 4 a frequency spectrum of an entire communications system accordingto the invention; and

FIG. 5 is a block diagram of the present transmission system.

DETAILED DESCRIPTION OF THE INVENTION

The formation of information channels is performed in the present systemsuch that no more than one duplex information channel per subscriberstation is prepared as needed, resulting in an economical constructionby adapting to the average traffic encountered. Both the base stationand the subscriber stations can be manufactured very cost-effectively.

The possibility of the transmission of signals from the base station tothe subscribers is to be provided, with limitations in some cases in theopposite direction.

In accordance with the invention and referring to FIGS. 1-5, informationis transmitted from a base station 20 to a number of independentsubscriber stations 22 by converting each information (e.g., telephone)signal 1, using suppressed carrier single-side band modulation, to anarrow-band signal (see FIG. 3). As shown in FIGS. 3 and 4, thenarrow-band signals are allocated in a frequency-multiplex scheme suchthat the frequency spectrums occupied by the signals are closelyadjacent one another (FIG. 3). The frequency-multiplex signals aremodulated on a carrier to a higher frequency range 7 using angle (phaseor frequency) modulation, and each subscriber station accesses no morethan one of the information channels, Information is transmitted fromeach subscriber station 22 to the base station 20 using narrow-bandradio channels 6 (see FIG. 4).

The base station 20 has a capacity of n narrow-band information channels1, one of which is shown in FIG. 1, to be used for the transmission ofanalog or digital information, e.g., for telephone traffic providing thepossibility of responding.

In the base station these n-channels are prepared at the transmittingend such that they form a frequency-multiplex signal 3, 4, as shown inFIG. 3. This multiplex signal is preferably imposed upon aradiofrequency carrier 9 (FIG. 4) by angle modulation and radiated froman antenna in a broad-band radio channel 7.

In the direction of transmission from the subscriber station to the basestation, the subscriber stations transmit in narrow-band channels 6 atdifferent frequencies. By using angle modulation, it is possible hereagain to minimize the requirements of the radiofrequency components.

The transmission of signal and control commands is performed, dependingon the size of the system, either by multiplexing in a normalinformation channel or in a special organization channel 2 (FIG. 2). Incomparison with known processes, this method has the advantage that onlyin the processing of the multiplex signal in the NT (signal frequency)and TF (carrier frequency) plane of the transmitter of the base stationis it necessary to make the greatest demands on the intermodulationlevels of the components and on the stability of the signal processing.Extremely linear radiofrequency amplifiers or complex combinationcircuits requiring high power and occupying much space are avoided inthis manner, as well as relatively expensive VHF or UHF synthesizers.

In the receivers of the subscriber stations there is also a shift in therequirements regarding stability under strong signals, linearity andhigh frequency stability in the radiofrequency section to the TF/NFplane.

The transmitters of the subscriber stations and the receiver in the basestation are not different from the corresponding components in knownradio transmission systems of this kind.

On account of the special electromagnetic wave propagation conditions inthe radiofrequency range interference problems can occur due tomultiple-path propagation, and they must be heeded especially in thecase of great bandwidths and in mobile operation with relatively loweffective antenna heights. Consequently, use is proposed substantiallyfor not excessively great numbers of channels and fixed subscriberstations.

A radio transmission system of this kind can be made such, according toone embodiment of the invention, that the channel allocation schemes inthe two direction of transmission are not identical, by providing thatin the transmitter part of the base station 20 signal processing isperformed as follows:

Each information channel 1 is put on a first modulator which produces asingle-band signal 3, 4 (FIG. 3) which is set to a specific frequencywithin the NF (signal frequency) and TF (carrier frequency) range,respectively. The individual information channels then have a setfrequency separation from one another (frequency allocation scheme) asshown in FIG. 3. They are then summed and form a frequency multiplexsignal.

If the information channels are, for example, analog telephone channelswith a bandwidth of 0.3 to 3.4 kHz 1 (see FIG. 1), then the resultantfrequency-multiplex signal can be arranged with a 4 KHz channel intervalbeginning at, e.g., 40 kHz. For 60 channels a multiplex signal bandwidthof up to 280 kHz would be needed.

Such signals are known, for example, in carrier frequency technology, oras baseband signals in radio beam technology.

The process for the production of these signals can be assumed to bewell known.

Special technical requirements exist in regard to the spectral purity ofthe multiplex signal by assuring very little interaction between theinformation channels. Intermodulation, undesirable mix products andnoise limit the maximum possible number of channels and the attainablesignal-to-noise ratio per channel, the limit varying with the cost andcomplexity of the circuitry.

The multiplex signal thus prepared is fed to a phase modulator orfrequency modulator so that a radiofrequency carrier is given awide-band modulation in comparison to the individual information channel9. Then the signal can be amplified to the required transmission powerin a not necessarily linear output stage and radiated from an antenna.The transmitted signal 9 occupies at least twice the bandwidth of themultiplex signal. See FIG. 4. Transmitter power and the occupiedradiofrequency band width can be exchanged with one another depending onthe given signal-to-noise ratio through the modulation index that is tobe selected.

Any kind of process heretofore known requires relatively expensive VHFor UHF synthesizers as well as one radio frequency (RF) amplifier perinformation channel, with the formation of the multiplex signal bybulky, expensive combination circuits requiring a substantially greaterRF power per channel, or else one antenna per channel.

Common to both processes is that the possible number of informationchannels is relatively small.

In the receiver of the subscriber station 22, the RF part needs tosatisfy relatively low requirements as regards linearity and frequencystability. These requirements need to be considered only in the case ofsignal demodulation and the selection/demodulation of the desiredinformation channel in the TF and NF planes.

If the amount of traffic per subscriber is limited in time, asubstantially greater number of subscribers can be served with this kindof radio transmission system than there are information channelsavailable (directional transmission, multiple access).

According to the invention, in the present radio transmission system, adigital organization channel 2 (FIG. 2) is present in the multiplexsignal for signaling and management or organization purposes internal tothe system, and for the defining the structure of connections or linksbetween the base station 20 and the subscriber stations 22. Theorganization channel is produced in a time-multiplex manner. Anotherdigital organization channel 10 is available for signaling from thesubscriber stations 22 to the base station 20. All subscriber stations22 may use the organization channel 10 as an independent channel intime-multiplex random access. The organization channels 2, 10 for thetwo directions of transmission need not be incorporated identically intothe channel scheme.

Thus, a digital organization or service channel 2 (FIG. 2) in thedirection from the base station to the subscriber stations is proposedfor the necessary organizational, signaling and control processes in thesystem. This organization channel is fed by a process control unit andcan be received by the subscriber apparatus, so that signaling at thestart of an information transmission, frequency assignments of freeinformation channels, system-internal reports etc. can be made availableto the subscribers.

The organization channel 2 (FIG. 2) in that case is locked into thefrequency-multiplex signal of the base station that is separate infrequency from the information channels (see FIG. 3), and transmittedafter RF modulation in a frequency band common with the informationchannels see FIG. 4, In the subscriber telephones the organizationchannel is available after signal demodulation.

In the direction from the subscriber stations to the base station, thecommunication of organization information takes place in an independentnarrow-band radio channel 10 (FIG. 4) which can have the same parametersas an information channel.

To prevent collision between transmissions from different subscriberstations on the subscriber organization channel, the individualsubscriber stations preferably operate in a time-muliplex manner.

In one embodiment of the present system, the base station organizationchannel (i.e., in the transmission direction from the base station 20 tothe subscriber stations 22) is a channel of lowest frequency among thechannel frequencies 7 transmitted by the base station, and thesubscriber station organization channel (in the transmission directionfrom the subscriber station 22 to the base stations 20) is at a middleportion of the frequency band allotted all of the subscriber stations22. See FIG. 4. This may be accomplished in the following way.

A pilot signal can be inserted into the multiplex signal of the basestation transmitter. This pilot signal can serve for various controlpurposes in the subscriber stations, such as signal-to-noise ratio,and/or as a control signal for the timing and timing correction, as wellas retuning the frequency of the subscriber stations. In this manner asimple synchronization of the entire system is possible. For anotherthing, the cost involved in achieving the required stability of thefrequency processing in the subscriber stations 22 is substantiallyreduced, since locking onto a highly stable calibration source in thebase station 20 can be performed. In one embodiment, a generator at thebase station 20 produces a pilot signal that is integrated into themultiplex base station signal, and the subscriber stations 22 use thepilot signal for control purposes, or for obtaining timing detection,timing correction, and retuning of the subscriber station frequencies.

The receiver sections of the subscriber stations are thus advantageouslydesigned so that a demultiplexer follows the demodulator for thewide-band received signal. This demultiplexer makes the organizationchannel permanently available at its output. Furthermore, the pilotsignal and, if required, an information channel is made ready at otheroutputs.

Such an arrangement permits receiving signals and commands parallel withthe actual information transmission and thus can effectively improve thequality of the system, e.g., for changing frequency in the case oftrouble in radio channels, for transmitting additional information, forsynchronization purposes etc. The circuitry is comparativelyinexpensive, so that only an RF input section and a demodulator arestill required for the entire receiver. Parallel processing is performedonly following the demultiplexer on the NF/TF plane. It is contemplatedthat in the present radio transmission system, the subscriber stations22 will have receivers for demodulating the wide-band base stationsignal, and a de-multiplexer which produces at its output a data signalcarried by the organization channel for signaling and control purposes,a freely selectable information channel, and the mentioned pilot signalfrom the base station. Such a subscriber receiver can also be used forthe data signal of the permanently received organization channel forpurposes of timing detection, timing correction, and for retuning thefrequency of the subscriber stations. What is required for this is ahighly stable data timing in the organization channel with anappropriate coding, the extraction of appropriate spectral lines servingas control data for the frequency processing of the subscriber stations.

Furthermore, it is contemplated that in the present radio transmissionsystem the subscriber stations 22 will use the data signal from theorganization channel for timing detection, timing correction, and forretuning of the subscriber station frequencies. Also, the base station20 and the subscriber stations 22 may be coupled with informationsources or sinks via radio or wire.

Accordingly, neither the base station nor the subscriber stations haveto be direct information sources or information sinks. The base stationcan be, for example, connected to the public telephone network viacables or directly coupled to an exchange. The subscriber stations canact, for example, as private branch exchanges or as a terminal forsubscriber end apparatus.

Likewise, transfers to other public or nonpublic radio or cable networksare possible.

The necessary information exchange, organization procedures, control andsignaling procedures can then be performed by a special process controlunit.

What is claimed is:
 1. A method of communicating information in a radiotransmission system including a base station and a number of independentsubscriber stations, comprising:converting first information signals tobe communicated from the base station to the subscriber stations tonarrow-band signals using single-side band suppressed carriermodulation; setting frequency band limits for each of said narrow-bandsignals to allow said narrow band signals to form a combined,frequency-multiplex signal; modulating said frequency multiplex signalon a carrier using phase or frequency modulation to produce a wide bandradio signal containing the first information signals to be communicatedfrom the base station to said subscriber stations; accessing, at eachsubscriber station, no more than one of said first information signals;and communicating second information signals from each subscriberstation to said base station by transmitting the second informationsignals within corresponding narrow-band channels.
 2. The method ofclaim 1, including allocating different radio frequency bands for saidwide band radio signal and for said narrow-band radio channels.
 3. Themethod of claim 1, includingtransmitting a first digital organizationchannel from the base station to the subscriber stations for internalsystem signaling and management purposes and for structuring linksbetween the base and the subscriber stations, forming the organizationchannel in a time-multiplex format and placing the organization channelas part of the combined, frequency-multiplex signal at the base station;providing a second digital organization channel for signaling from thesubscriber stations to the base station such that each subscriberstation can randomly access the second organization channel according toa time-multiplex scheme; and allocating the first and the secondorganization channels at different radio frequencies.
 4. The method ofclaim 3, including allocating said second organization channel as lowestfrequency channel among said narrow band radio channels, and allocatingsaid first organization channel at a center frequency within said wideband radio signal.
 5. The method of claim 1, includinggenerating a pilotsignal at said base station and integrating the pilot signal into saidfrequency-multiplex signal, and using the pilot signal at the subscriberstations for at least one of controlling station functions, detectingtiming, correcting timing, and retuning the frequencies of thesubscriber stations.
 6. The method of claim 5, includingdemodulating thewide-band signal at the subscriber stations using a receiver and ademultiplexer, and producing at an output of the demultiplexer a datasignal corresponding to said first organization channel for (a)signaling and controlling subscriber station functions, (b) selecting aninformation channel, and (c) providing said pilot signal.
 7. The methodof claim 6, including using the data signal at the subscriber stationsfor detecting timing, correcting timing, and for retuning thefrequencies of the subscriber stations.
 8. The method of claim 1,including connecting with information sources or sinks at the base andthe subscriber stations by radio or by wire.